XEmacs had outl-mouse that used outline-minor-mode plus a few pixmaps and mouse keybindings to let old lazy guys to hide and show (in my use case) sections and subsections etc of my AUCTeX buffers, ...

I'm hacking together an extension which will display a sidebar with an outline of whatever document you're working on. It was initially written to support writing in markdown-mode, but the technique I'...